Whether you have five or 50 yoga classes under your capris, Chair pose (aka Fierce) is one of the many basic postures you'll strike just about as often as Down Dog since it's the first pose in Sun Salutation B. As a yoga instructor, I've noticed some major no-nos that you'll want to avoid to prevent discomfort and to make sure you're getting the most out of the pose.
When you bend your knees and lower your hips into the pose, pretend you're about to sit down in an imaginary chair. This will remind you to keep your body weight back into your heels. Give your toes a little wiggle to make sure you're shifting toward the back of your feet and to release tension in your forefoot. Shifting back into your heels works your glutes and thighs more and will also prevent strain on the knees.
You can do this pose with your raised arms shoulder-width distance apart or with palms pressed together, but whichever variation you choose, be sure your shoulders are relaxed and sliding back and down away from your ears to alleviate any tension. If you're not sure if they're relaxed, purposely scrunch them up toward your ears, and then actively release them down. You want to create as much space in your neck and shoulders as possible, so do this pose with your arms by your sides if raising your arms creates tension in your neck, shoulders, and upper back.
Sticking Out Your Tail
If your lower back aches in this pose, bring your awareness toward your seat. Actively tuck your tailbone in and engage your abs by pulling your navel toward your spine. This action will help elongate your spine, which allows you to breathe more deeply.
When doing Chair pose, you want to squat low enough, bringing your thighs as parallel to the floor as you can handle. It's OK if your knees project out over your feet, but shifting your weight back into your heels will ensure they don't go too far forward to cause knee pain. It's a really common alignment mistake not to sit low enough, but when you do, you'll more effectively strengthen your quads, hamstrings, and glutes.